Study On Damage Deterioration Timeliness Of Mechanical Properties Of Intermittent Jointed Sandstone Under Water-rock Interaction

In underground engineering,there are often one or more irregular joints in the underground rock mass of coal mine.The mechanical properties such as strength of jointed rock mass under long-term water-rock action have obvious deterioration effect.When the stress exceeds the ultimate stress,the crack initiation,expansion and penetration will occur in the rock mass,which will eventually lead to the overall instability and failure of the rock mass and cause coal mine dynamic disasters.In this paper,the intermittent jointed sandstone with different joint dip angles and connectivity rates is taken as the research object,and the uniaxial compression and acoustic emission experiments under different immersion time are carried out.Based on the uniaxial compression experimental data,a numerical simulation model was constructed.The PFC^2D simulation software was used to conduct biaxial compression experiments on sandstone with different dip angles and lengths of joints under water-rock interaction.The mechanical parameters,crack propagation and damage evolution process of sandstone with different joint dip angle,connectivity rate and length under different soaking time under load are explored.The results show that:（1）Under the action of water rock,with the increase of immersion time,the compressive strength decreases gradually.In the same immersion time,with the increase of joint dip angle,the compressive strength shows a“V”type change.When the joint dip angle is 60°,the compressive strength is the smallest,and the compressive strength of the complete rock sample is greater than that of the joint.By analyzing the experimental data of numerical simulation,the variation law consistent with the indoor uniaxial compression experiment is obtained.（2）With the increase of immersion time,the elastic modulus decreases gradually,and the elastic modulus of jointed sandstone with joint dip angle of 60°is the smallest.The variation law of elastic modulus obtained by biaxial simulation is the same as that of indoor uniaxial experiment.With the increase of joint length,the compressive strength,elastic modulus and pre-peak energy of sandstone decrease,and the joint length is negatively correlated with compressive strength,elastic modulus and pre-peak energy.（3）The existence of joints can change the failure mode of rock mass,and the failure modes of fracture specimens with different dip angles are also different.This leads to the anisotropy of deformation strength parameters in the failure process of intermittent jointed sandstone.With the increase of joint inclination angle,the failure mode evolution process of rock samples is as follows:from tensile failure（0°,30°）,composite tensile-shear failure（45°）through the joint end,gradually developed to shear supplemented tensile failure（60°）,tensile failure（75°）,and finally developed into splitting tensile-shear failure（90°）.The joint connectivity rate can completely change the damage degree of rock mass specimens.The larger the connectivity rate,the more serious the damage under uniaxial compression and the lower the compressive strength.Through the comparative analysis of indoor uniaxial experiment and biaxial numerical simulation experiment,it is found that the effect of confining pressure on compressive strength,elastic modulus and other numerical values is improved.（4）Based on the uniaxial compression test data,the damage constitutive model of sandstone with different joint dip angles under water-rock interaction is established by using damage mechanics,and the accuracy of the model is verified by comparing with the experimental data.In the early stage of immersion,the damage variable changes greatly,and when the moisture content of the specimen changes slowly,the damage change also slows down.